From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-10.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2CA7FC43331 for ; Wed, 25 Mar 2020 11:13:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E7CDE20772 for ; Wed, 25 Mar 2020 11:13:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585134834; bh=BzCuAlwuxAGX3F005NMoJ1SQbjE8q7I0TQauHBZ+31o=; h=From:To:Cc:Subject:Date:List-ID:From; b=XWz8iES/MEedE8ksX+XIdyw9a9KlAr+M8YBVWui8Ri0j0qt6ry8Z1tqz+YRJxqpK/ lXeYEWthJ2Vqv8Be1xoldmjI6Rvoxo7ZB6X4WF0oY6Y4QNNZ+YIf3Ba4ASIUnSWPGa ADsiqtiYXcY9t09R5i7P3gujIshwOlfYIpRWyGXM= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727177AbgCYLNw (ORCPT ); Wed, 25 Mar 2020 07:13:52 -0400 Received: from mail.kernel.org ([198.145.29.99]:56952 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726139AbgCYLNw (ORCPT ); Wed, 25 Mar 2020 07:13:52 -0400 Received: from localhost.localdomain (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id DEA2B20714; Wed, 25 Mar 2020 11:13:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585134832; bh=BzCuAlwuxAGX3F005NMoJ1SQbjE8q7I0TQauHBZ+31o=; h=From:To:Cc:Subject:Date:From; b=YKnz9Hecl5fxuD2C7GFv4OgVyH5qObz8ssS4YRbT+X2qm4uFTkze2Xy/WOTKAIx4p MbacL9p1KQ5APanJBD1/IcPOu7Ub8XAddFVr6IVWbskaNV8/HkmR6cCGJGgDkaVzFe G5G4M+ftNeePFy0vNPDu5KMBY8SVoDgdZvQcZT4c= From: Will Deacon To: linux-kernel@vger.kernel.org Cc: linux-arm-kernel@lists.infradead.org, kernel-team@android.com, Will Deacon , Catalin Marinas , Linus Torvalds Subject: [PATCH] mm/mremap: Add comment explaining the untagging behaviour of mremap() Date: Wed, 25 Mar 2020 11:13:46 +0000 Message-Id: <20200325111347.32553-1-will@kernel.org> X-Mailer: git-send-email 2.20.1 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Commit dcde237319e6 ("mm: Avoid creating virtual address aliases in brk()/mmap()/mremap()") changed mremap() so that only the 'old' address is untagged, leaving the 'new' address in the form it was passed from userspace. This prevents the unexpected creation of aliasing virtual mappings in userspace, but looks a bit odd when you read the code. Add a comment justifying the untagging behaviour in mremap(). Cc: Catalin Marinas Reported-by: Linus Torvalds Signed-off-by: Will Deacon --- mm/mremap.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/mm/mremap.c b/mm/mremap.c index af363063ea23..d28f08a36b96 100644 --- a/mm/mremap.c +++ b/mm/mremap.c @@ -606,6 +606,16 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len, LIST_HEAD(uf_unmap_early); LIST_HEAD(uf_unmap); + /* + * There is a deliberate asymmetry here: we strip the pointer tag + * from the old address but leave the new address alone. This is + * for consistency with mmap(), where we prevent the creation of + * aliasing mappings in userspace by leaving the tag bits of the + * mapping address intact. A non-zero tag will cause the subsequent + * range checks to reject the address as invalid. + * + * See Documentation/arm64/tagged-address-abi.rst for more information. + */ addr = untagged_addr(addr); if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE)) -- 2.25.1.696.g5e7596f4ac-goog